The need for a non-invasive technique for measuring the pressure in body cavities of animals or humans is recognized as highly desirable for continuous or intermittant monitoring of body conditions. Such cavities as the cranium, vena cava, bladder and others provide valuable and sometimes critical information for maintaining the well-being or survival of an animal or human. For example, it is known that the intracranial pressure provides a valuable indication of well-being for a variety of clinical conditions, including shock trauma and hydrocephalus.
Present-day pressure sensors are intended for permanent implantation for non-invasive utilization of their pressure sensing function. They do, however, constitute a foreign body whose presence may create physical problems which must be obviated for proper functioning of the sensor and for a minimum of discomfort and disfigurement to the patent. For instance, it is highly desirable that the component parts of the sensor external to the body portion being monitored be firmly and securely mounted as any movement of such component parts, particularly when mounted subcutaneously can produce irritation and attendant discomfort to the patient. It is not uncommon for body tissue such as that produced by surgical scars during implantation or muscular growth to interfere with the proper operation of the sensor by imposing pressures on those parts of the device which are designed to be responsive to pressures such as ambient pressure. When such a sensor is used particularly for pressure sensing of the intracranial cavity, the pressure transmitting fluid conduit communicating with the pressure sensing device in the cavity can easily impose pressure on the brain, the sensitive tissue of which can be easily damaged. Furthermore, since such pressure sensing apparatus should be devoid of deterioration during its useful life, which may be for a period of many years, it is highly desirable that the component parts by completely compatible and/or inert chemically and physically with the adjacent body portions so as to avoid any adverse reactions. Additionally, the component parts of the apparatus and pressure transmitting fluid which are in contact should be completely compatible so as to avoid any deteriorating reaction therebetween. In addition, leakage between the component parts is to be avoided, but should there be a failure, the fluid should be compatible in the event of leakage. Furthermore, the parts should remain in perfect working order throughout the useful life of the apparatus. Since such a pressure sensor uses radioactive material, such radioactive material should be of the type which requires no replacement during the useful life of the sensor and it should function in a manner which is non-injurious to the body and to provide a radioactive output in a highly accurate and uniform manner so as to reflect with extreme precision, the sensed body fluid pressure throughout its range of operation under both positive and negative fluid pressures.